Remove tower from axum's public API (#229)

Instead rely on `tower-service` and `tower-layer`. `tower` itself is
only used internally.

Fixes https://github.com/tokio-rs/axum/issues/186

src/tests/mod.rs

@@ -1,5 +1,6 @@
 #![allow(clippy::blacklisted_name)]
 
+use crate::BoxError;
 use crate::{
     extract,
     handler::{any, delete, get, on, patch, post, Handler},
@@ -23,7 +24,8 @@ use std::{
     task::{Context, Poll},
     time::Duration,
 };
-use tower::{make::Shared, service_fn, BoxError, Service};
+use tower::{make::Shared, service_fn};
+use tower_service::Service;
 
 mod get_to_head;
 mod handle_error;

src/tests/or.rs

@@ -1,10 +1,8 @@
+use super::*;
+use crate::{extract::OriginalUri, response::IntoResponse, Json};
 use serde_json::{json, Value};
 use tower::{limit::ConcurrencyLimitLayer, timeout::TimeoutLayer};
 
-use crate::{extract::OriginalUri, response::IntoResponse, Json};
-
-use super::*;
-
 #[tokio::test]
 async fn basic() {
     let one = Router::new()